In most subsea applications electric-hydraulic valves are used to control operation of equipment such as process valves (opening and closing) and actuators. The valves are either operated with electric power supplied directly from the surface in separate electric wires, or by means of electric power in wires from a local subsea control system. The valves can be installed inside a subsea container (pod) together with the subsea control system, or some distance from the control system, normally also in a dedicated container. The main purpose of the container is to provide a benign atmosphere for the valve bodies, and the container is therefore normally filled with a fluid with electric isolation and corrosion protection, typically a hydraulic fluid or a silicone oil.
The liquid inside the container is normally maintained at the same pressure as the external ambient pressure due to the water depth. Typically, at 3.000 meter water depth the ambient pressure due to the depth is about 300 bar. The pressure inside the container is then normally maintained at approximately 300 bar as well, using pressure compensating devices. Pressure compensators are typically bladders that can expand or contract to compensate for minor changes in fluid volume inside the container due to temperature or absolute pressure changes.
As the water depth increase the external over pressure outside the valve body will increase. If the fluid that the valve is controlling needs to be kept at low pressure, the difference between the controlled fluid pressure and the ambient pressure surrounding the valve body will increase. For instance, at 5.000 meter water depth the external pressure can be about 500 bar and if the pressure of the liquid is say 100 bar then the pressure differential that the valve need to operate at is 400 bar. This high pressure differential may be a challenge for existing, qualified valves.
It is common to pressure test subsea piping systems to check the systems for leakage and a test pressure that is often used is 10.000 psi, or some 690 bar. The test pressure is applied in addition to the static pressure at depth. At very deep water the absolute pressure during pressure testing can then be very high, typically 1.000 bara at 3.000 meter water depth.
The electric-hydraulic valves are sometimes used to control a barrier fluid that is used inside equipment typically such as electric motors. The barrier fluid is kept at the test pressure plus a small margin to ensure a positive over pressure, typically 20-30 bar above the test pressure.
This means that in a case at say 3.000 meter water depth with say a test pressure of 10.000 psi the liquid that the electric-hydraulic valve shall control will be kept at 700+300+30=1.030 bar pressure on the inlet/outlet ports.
If a electric-hydraulic valve in such a case is used and installed inside a chamber where the pressure is balanced against the external water pressure at depth, the pressure differential between the external valve body and the liquid it is controlling can be very high, typically 1.030−300=730 bar. This high pressure differential may be a challenge for existing, qualified valves.